The optical material contrast of single nanoparticles depends on both, the dielectric properties of the nanoparticles and on their size. The topography image (gray) shows the homogeneous distribution of nanoparticle adsorbed on the surface of a Si substrate, where the particle size ranges from approximately 5-20nm. Detailed analysis of the near-field signal (colored image) allows to separate the particles according to the refractive index. Au particles yield higher signal and corresponding pixels are marked red in the topography image compared to PS particles which exhibit a lower signal (marked green). The results of this study allow to establish material-specific mapping of polydisperse nanoparticle mixtures with nanoscale spatial resolution. Possible applications range from the material-specific mapping of nanoparticle assemblies to the measurement of the doping concentration in single semiconductor nanoparticles.

Further reading:

• A. Cvitkovic, N. Ocelic, and R. Hillenbrand, “Material-Specific Infrared Recognition of Single Sub-10 nm Particles by Substrate-Enhanced Scattering-Type Near-Field Microscopy”, Nano Letters 7, p.3177 (2007)
• A. Cvitkovic, N. Ocelic, J. Aizpurua, R. Guckenberger, and R. Hillenbrand, “Infrared Imaging of Single Nanoparticles via Strong Field Enhancement in a Scanning Nanogap”, Phys. Rev. Lett. 97, p.60801 (2006)